Method for forming a controlled impedance flex circuit

ABSTRACT

A system and method for providing a controlled impedance flex circuit includes providing an insulative flexible substrate having opposed first and second surfaces and having through holes extending from the first surface to the second surface. A pattern of conductive traces is formed on the first surface of the flexible substrate. A film of conductive adhesive is applied to the second surface and to the through holes. The through holes are aligned to contact ground traces in the pattern of conductive traces on the first surface. Thus, a ground plane is established for creating an environment for high frequency signal propagation. The conductive adhesive may be a b-stage epoxy or a thermoplastic material. In the preferred embodiment, a tape automated bonding frame is fabricated.

This is a continuation of application Ser. No. 08/431,158 filed on Apr.28, 1995, now abandoned which is a divisional of 08/169,823 now Pat. No.5,448,020 filed Dec. 17, 1993.

TECHNICAL FIELD

The present invention relates generally to flexible circuits such astape automated bonding frames and more particularly to methods andsystems for controlling the impedance of flex circuits.

BACKGROUND ART

One challenge confronting circuit designers in the electronics industryis finding more efficient and reliable methods of electricallyconnecting an integrated circuit chip to other circuitry. One techniqueis referred to as tape automated bonding, or "TAB." This interconnectiontechnique utilizes a continuous insulative tape which is similar tophotographic film to provide a planar substrate for chips that areattached to individual frames of the tape. A spider-like metal patternof conductive traces is etched or punched from a metal foil on eachframe. The traces may "fan out," i.e. radiate from the center of theframe to the edges, or may be sets of parallel lines. A chip iscarefully aligned with the center of the frame, so that contacts of thechip are precisely located at corresponding conductive traces in thecentral portion of the frame. The chip is then attached to the TABframe. The connection of the chip contacts to the inner portion of theframe is referred to as "inner lead bonding." The electrical performanceof a single-metal TAB device is limited by parasitic inductance andcapacitance of the assembly of traces. The parasitic properties limitthe frequency bandwidth of the TAB device and reduce the signal-to-noiseratio.

A technique for reducing parasitic inductance and capacitance is to forma ground plane on the side of the TAB frame opposite to the conductivetraces. The resulting "double-metal TAB device" is better equipped tomeet the requirements of high speed digital and high frequency analogapplications.

The ground plane of a double-metal TAB device must be electricallyconnected to establish ground potential. This can be done by the use ofthrough holes that extend through the insulative tape from a metal foilthat is used as the ground plane to ground traces of the pattern ofconductive traces on the opposite side. The through holes are filledwith metal. A second method of grounding the metal foil that forms theground plane is described in U.S. Pat. No. 5,142,351 to Matta, which isassigned to the assignee of the present application. The controlledimpedance TAB frame of Matta is formed without through holes. Instead,the metal foil that forms the ground plane is etched to include leadsthat are cantilevered from the insulative tape. The leads may then beattached to a grounded contact of the assembly to which the TAB frame iselectrically connected.

While the double-metal TAB devices perform well as controlled impedanceassemblies, in many circuit applications the assembly is costprohibitive. The double-metal version of a TAB frame may have a costthat is four times that of the single-metal version. The substantialincrease in cost to form a double-metal assembly applies equally to thefabrication of other controlled impedance flex circuits, such as thoseused as flexible printed circuit boards and flexible circuits formulti-chip modules.

What is needed is a method of forming an interconnection system forachieving a controlled impedance of a flexible circuit in a high yieldand economical manner.

SUMMARY OF THE INVENTION

The present invention provides a controlled impedance interconnectionsystem and a high-yield fabrication method by utilizing an electricallyconductive adhesive both as a ground plane on a side of a flexiblesubstrate opposite to a pattern of traces and as material to formelectrical paths through the flexible substrate. Within the pattern oftraces are one or more ground traces. A single application of theconductive adhesive forms the ground plane on a second surface of thesubstrate, fills substrate through holes to the ground traces on a firstsurface of the substrate, and optionally presents a bondable surface forattachment to a second substrate.

Typically, the flexible substrate is tape of a tape automated bondingframe. However, the flexible substrate may also be of the type employedfor flexible printed circuit boards or multi-chip module substrates. Anarray of holes is formed through the substrate. The positioning of thethrough holes is designed to ensure proper grounding along the entiretyof the ground plane to be formed.

The pattern of conductive traces on the first surface of the flexiblesubstrate may be formed using photolithographic techniques. For example,a metal foil may be attached to the first surface and the foil may beetched to remove selected portions of the metal. The remaining portionsdefine the pattern of traces. For a tape automated bonding frame, outerlead ends are cantilevered from the substrate following completion ofthe fabrication procedure.

The ground plane may be formed either before or after the patterning ofthe conductive traces. The electrically conductive adhesive is appliedto the second surface of the flexible substrate. The adhesive fills thethrough holes for connection to ground traces on the first surface.

For many applications of the present invention, only the electricalproperties of the adhesive will be important after the conductiveadhesive has penetrated the through holes and has been fixed to thesurface of the flexible substrate. In these applications, the conductiveadhesive is typically a conventional conductive epoxy that is cured in asingle step. Other applications may set demands for certain adhesiveproperties even after the conductive adhesive has been properly appliedto the flexible substrate. For example, the conductive adhesive may alsobe employed to fix a second substrate to the flexible substrate. Forthese applications, the conductive adhesive is preferably a b-stageepoxy or a thermoplastic adhesive which can be partially cured on theflexible substrate prior to the attachment of the second substrate.

In addition to acting as a ground plane, the conductive adhesive may bepatterned to include an electrically isolated sector that is connectedby means of a through hole to a trace designed to connect to a powersource, such as V_(cc). That is, the conductive adhesive may be used asa constant-potential utility plane other than ground.

An advantage of the present invention is that a high frequency,controlled impedance signal propagation environment is formed in acost-efficient fashion. The conductive adhesive may be applied in amanner compatible with reel-to-reel processing for fabricating flexcircuits, such as tape automated bonding frames. Typically, theconductive adhesive is applied using screen printing techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a tape automated bonding frame having throughholes in accordance with the present invention.

FIG. 2 is a partial, side sectional view of the frame of FIG. 1 having aconductive trace formed thereon.

FIG. 3 is a side sectional view of the structure of FIG. 2 having aconductive adhesive deposited on a back surface and into the throughhole.

FIG. 4 is a side sectional view of the structure of FIG. 3 having asecond substrate and a second pattern of conductive traces.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, a portion of a tape automated bonding (TAB)flexible substrate 10 is shown. The flexible substrate includes a firstseries of sprocket holes 12 near one edge and a second series ofsprocket holes 14 near the opposite edge. The sprocket holes are used toprecisely move the substrate relative to fabrication equipment, notshown.

Four rectangular openings 16, 18, 20 and 22 define a TAB frame. As willbe explained more fully below, an array of conductive traces is formedwithin the TAB frame. Typically, the ends of the traces will extendacross the rectangular openings 16-22, but the ends are excised prior touse. Thus, the outer trace ends of the conductive traces will becantilevered from the inner edges of the rectangular openings.

At the center of the frame defined by the rectangular openings 16-22 isa square opening 24. The square opening and the rectangular openings aretypical of TAB processing, but are not critical to the presentinvention. Moreover, the opening 24 at the center of the frame may havea different geometric shape. Typically, the size and the shape of thecentral opening is dictated by the integrated circuit chip or otherdevice to which the TAB frame is to be attached.

Precisely located through holes 26, 28 and 30 are formed in the flexiblesubstrate 10. The design of the array of through holes 26-30 is aimed atensuring uniformity of the electrical potential of a utility plane to beformed on the opposite surface of the flexible substrate 10. The throughholes may be formed in the same manner as the rectangular openings 16-22and the central square opening 24. For example, a punching operation oretching techniques may be utilized to form these features.Alternatively, the through holes may be generated by a laser.

The flexible substrate 10 conventionally employed in TAB operations is apolyimide substrate. However, other dielectrics may be substituted. Informing conductive traces on the surface of the flexible substrate, aconductive layer is plated or laminated onto the surface. For example, acopper layer having a thickness of approximately 33 μm may be formed.While not critical, a thin layer of gold may be plated onto the tracesto provide a desired bondability. The composition of the metal structureis not critical to the present invention.

The metal on the first surface is then patterned to provide conductivetraces 32 such as the one shown in FIG. 2. Patterning may be carried outby photolithographic techniques that include etching away selectedportions of the metal to leave a pattern of conductive traces. Theconductive traces radiate from the central square opening 24 and extendacross the four rectangular openings 16-22. The traces will includeground traces which will be held at ground potential during use of theTAB frame. At least some of the ground traces will extend across thethrough holes 26-30 of FIG. 1. The conductive trace 32 of FIG. 2 is onesuch ground trace, extending over through hole 26.

The pattern of traces 32 has inner lead ends 34 that are cantileveredfrom the flexible substrate 10. Outer lead ends extend beyond therectangular opening 22. However, when the TAB frame is excised from theremainder of the flexible substrate, the outer lead ends are alsocantilevered.

Referring now to FIG. 3, a conductive adhesive 36 is then applied to thesurface of the flexible substrate 10 opposite to the ground trace 32.Preferably, the conductive adhesive is applied using a screen printingmethod. Silkscreening is compatible with conventional processing steps.Silkscreening is sometimes used to form a solder mask atop theconductive traces of a tape automated bonding frame. While applying theconductive adhesive 36 using a syringe is not advantageous to massproducing TAB frames, syringe applications may also be used.

The conductive adhesive coats the back surface of the flexiblesubstrate. The conductive adhesive also extends into the through hole toform a via 38 that contacts the ground trace 32. Thus, the conductiveadhesive acts as a ground plane to reduce parasitic inductance andcapacitance of the pattern of conductive traces.

The material for forming the conductive adhesive 36 may be formulatedbased on a conventional epoxy, a b-stage epoxy, or a thermoplasticadhesive. A b-stage epoxy or a thermoplastic adhesive is preferred inembodiments in which the structure of FIG. 3 is to be attached toanother substrate. An acceptable b-stage adhesive is sold by Ablestick,Inc. under the trademark ABLEBOND 941-3. Acceptable thermoplasticadhesives are sold by Alpha Metals, Inc. under the trademarks STAYSTIK581 and STAYSTIK 501. The conductive adhesive 36 is applied in a fluidcondition, so that the via 38 is formed merely by the adhesive flowinginto the through hole. The adhesive is then cured to fix the material inplace. Typically, curing is accomplished by raising the temperature tocause cross-linking. If a b-stage epoxy or a thermoplastic adhesive isused, the material may be partially cured to facilitate shipping andhandling. Following handling, the second substrate can be attached andthe curing can then be completed.

The controlled impedance flex circuit of FIG. 3 provides a highfrequency signal propagation environment in a manner that is compatiblewith conventional reel-to-reel processing for fabricating tape automatedbonding frames. Consequently, the overall process flow is neithercumbersome nor prohibitively expensive.

Optionally, the tape automated bonding frame of FIG. 3 may be combinedwith additional patterns of conductive traces. Referring now to FIG. 4,in addition to functioning as a ground plane, the conductive adhesive 36binds a second flexible substrate 40 to the flexible substrate 10,whereby the ground plane is shared between the two substrates.Preferably, the second substrate also includes an application ofconductive adhesive which flows into a through hole to form a via 44 toa conductive trace 42 on the opposite side of the second substrate. Thetwo applications of the conductive adhesive are then joined together.Any number of alternating layers of traces, substrate, and conductiveadhesive ground plane may be formed in the same manner.

The present invention has been described and illustrated with referenceto tape automated bonding frames. The use of a film of conductiveadhesive to form a ground plane for a flex circuit may also be used forfabricating other controlled impedance flex circuits, such as flexibleprinted circuit boards and flexible substrates of a multi-chip module.

In FIGS. 2 and 3, the ground trace 32 is patterned prior to theapplication of the conductive adhesive 36. Alternatively, the patterningof the conductive traces may take place subsequent to depositing theconductive adhesive. For example, a copper foil may be applied to thecircuit side of the substrate 10, whereafter screen printing of theconductive adhesive occurs prior to patterning of the copper foil.

In another alternative embodiment, the via 38 of FIG. 3 may be alignedto contact a trace which is at a potential other than ground. That is,the conductive adhesive may function as a utility plane that is held ata constant potential such as 5 volts. The conductive adhesive may bepatterned to include electrically isolated planes, with one plane beingelectrically connected to ground and a second plane being connected to avoltage source.

I claim:
 1. A method of fabricating a flexible circuitcomprising:forming through holes in a flexible substrate having opposedfirst and second surfaces; and establishing a controlled impedancecircuit on said flexible substrate, including simultaneously applying anelectrically conductive adhesive on said second surface and into saidthrough holes to form a structurally and electrically unitary layerfilling said through holes and on said second surface, establishing saidcontrolled impedance circuit further including forming a pattern ofconductive traces on said first surface such that selected traces insaid pattern contact said conductive adhesive in said through holes,thereby electrically interconnecting said selected traces and saidconductive adhesive on said second surface and establishing a generallyconstant-voltage plane on said second surface.
 2. The method of claim 1wherein forming said pattern of conductive traces includes applying aunitary metallic foil to said first surface of said flexible substrateand includes one of punching and etching to remove selected portions ofsaid metallic foil, leaving an array of metal leads extending along saidfirst surface.
 3. The method of claim 2 wherein forming said pattern ofconductive traces includes applying said metallic foil and removing saidselected portions such that said array of metal leads has endscantilevered from said flexible substrate.
 4. The method of claim 2wherein applying said electrically conductive adhesive is a step ofscreen depositing epoxy.
 5. The method of claim 1 wherein applying saidconductive adhesive includes forming a b-stage epoxy to said secondsurface and includes partially curing said b-stage epoxy at an elevatedtemperature.
 6. The method of claim 1 wherein applying said conductiveadhesive includes forming a thermoplastic adhesive to said secondsurface and includes partially curing said thermoplastic adhesive at anelevated temperature.
 7. A method of fabricating a tape automatedbonding frame comprising:providing a flexible substrate; forming apattern of holes through said flexible substrate; applying a metallicfoil on a first surface of said flexible substrate; removing selectedportions of said metallic foil, thereby leaving an array of conductiveleads, including ground leads extending over said holes; and forming alayer of conductive adhesive on a second surface of said flexiblesubstrate and simultaneously filling said holes with said conductiveadhesive to contact said ground leads, thereby forming a controlledimpedance tape automated bonding frame, said conductive adhesive being astructurally and electrically unitary layer that fills said holes andcovers said second surface.
 8. The method of claim 7 wherein removingselected portions of said metallic foil leaves outer ends of saidconductive leads extending beyond said flexible substrate of said tapeautomated bonding frame.
 9. The method of claim 7 wherein said removingselected portions of said metallic foil includes using photolithographictechniques.
 10. The method of claim 7 further comprising utilizing saidconductive adhesive to secure a second array of conductive leads to saidcontrolled impedance tape automated bonding frame.
 11. The method ofclaim 7 further comprising attaching an integrated circuit chip to saidarray of conductive leads.
 12. A method of fabricating a flexiblecircuit comprising:forming through holes in a flexible substrate havingopposed first and second surfaces; establishing a controlled impedancecircuit on said flexible substrate, including simultaneously applying anelectrically conductive adhesive on said second surface and into saidthrough holes to form a structurally and electrically unitary layerfilling said through holes and on said second surface, establishing saidcontrolled impedance circuit further including forming a pattern ofconductive traces on said first surface such that selected traces insaid pattern contact said conductive adhesive in said through holes,thereby electrically interconnecting said selected traces and saidconductive adhesive on said second surface and establishing a generallyconstantvoltage plane on said second surface; and attaching a secondsubstrate by joining a first side of said second substrate to saidconductive adhesive, said second substrate having a second side having asecond pattern of conductive traces, wherein applying said conductiveadhesive includes providing a conductive epoxy and partially curing saidepoxy on said flexible substrate prior to attaching said secondsubstrate and further includes executing a second cure of said epoxy tofix said second substrate to said flexible substrate, said epoxy therebyproviding a generally constant-voltage plane for each of said flexibleand second substrates.
 13. The method of claim 12 further comprisingforming through holes into said second substrate prior to attaching saidsecond substrate and filling said through holes with said epoxy duringattachment of said second substrate to said flexible substrate, saidsecond substrate being a second flexible substrate.